There is a known thermomagnetic recording system which uses a layer of soft magnetic material having an easy axis of magnetization normal to the layer surface on which the thermomagnetic recording is performed by irradiating a laser light thereon. The system is disclosed in the publicated document of Japanese Kokai No. 158005/1982 (Japanese patent application, No. 45232/1981) filed by the present applicant. The layer of soft magnetic material having an easy axis of magnetization normal to the layer surface used therein is a layer made of soft magnetic material, for example, YSmCaFeGe-series garnet such as (YSmCa).sub.3 (FeGe).sub.5 O.sub.12 or the like, having uniaxial magnetic anisotropy strong in the direction normal to the layer surface and an easy axis of magnetization normal to the layer surface. The layer of soft magnetic material having an easy axis of magnetization normal to the layer surface is required to have such soft magnetic property that when this layer is used as a recording medium, the diameter of information bits to be written therein is practically determined only by a bias magnetic field, and the coercive force thereof is desired to be less than about 3 Oe, preferably about 1 Oe or below. This layer of soft magnetic material having an easy axis of magnetization normal to the layer surface is formed by growing on a crystalline substrate of rare-earth gallium garnet such as non-magnetic gadolinium gallium garnet (GGG) by the liquid phase epitaxial (LPE) growth of YSmCaFeGe-series garnet crystal or the like. The writing an information on that layer is carried out such that a bias magnetic field of a predetermined strength is first applied to the layer of the soft magnetic material so as to make the layer magnetized to have a single magnetic domain over the whole layer surface thereof and magnetization directed to the direction perpendicular to the layer surface. Then, under this state, when an optical pulse is incident on the layer surface under being focussed thereon, this light energy is converted to heat energy so as to generate heat by which bits (magnetic bubbles) are formed, or the writing of an information on the layer can be carried out. The bits thus written are cylindrical magnetic domains each having a predetermined diameter and the magnetization directed to the direction opposite to the applied bias magnetic field.
The strength of the bias magnetic field applied to the layer of soft magnetic material so as to produce a single magnetic domain therein over the whole layer surface thereof is selected in a range from the run-out magnetic field to the collapse magnetic field of the material, for example, between 57 Oe and 73 Oe for the LPE layer of (YSmCa).sub.3 (FeGe).sub.5 O.sub.12. Because the coercive force, particularly magnetic wall coercive force of the layer of soft magnetic material is extremely small and hence the bias magnetic field applied thereto can be small, the bias magnetic field generating means may be a small solenoid coil, rubber magnet or the like.
The reading-out of the information from the magnetized layer on which the recording was made as above is carried out such that a light, for example, a laser light is linearly polarized by a polarizer and then irradiated on the recorded medium. When the linearly polarized light passes through the magnetized layer, it is subjected to the rotation by the Faraday-effect. Then this rotated light is applied through an analyzer into a photo-detecting means so that the output corresponding to the information bits is produced therefrom and hence the reading-out of the information is performed.
According to the above thermomagnetic recording on the LPE magnetic garnet thin film, since its magnetic wall coercive force Hc is as small as about 1 Oe or below, the movement of information becomes possible which could not be carried out by the prior art thermomagnetic recording, namely, Curie point writing, compensation point writing and so on. The movement of information is to move the recorded bit, namely, the magnetic bubble domain recorded at a certain position to other position to thereby give logic calculation function. As a method for carrying out the movement of information bits, there is a method disclosed in the publicated document of Japanese Kokai No. 17505/1983 of the present applicant. The method for moving information bits disclosed therein is such a method that a new bit independent from an information bit, namely, a cue bubble is produced by the radiation of continuous light and the repulsion force between this cue bubble and the magnetic bubble of the already recorded information bit is used to move the information bit. That is, in this method, in order to move a desired information bit from a track along which the information bits are aligned, while applying a bias magnetic field between the run-out magnetic field and the collapse magnetic field to the recorded medium, a substantially continuous light is made incident on a portion of the medium apart from the track by a predetermined distance and the cue bit, which is produced in association with the continuous light, causes the information bit to be moved, which comes near to the cue bubble due to the temperature gradient generated by the irradiation of light, to be repelled outside the track by a repulsive force therebetween. The substantially continuous light mentioned here includes an intermittent light of such an extent that no recording is left even by the irradiation thereof, in addition to the ordinary continuous light and may be any light if a pulse repetition of a beam is enoughly faster than a relative speed between a magnetic thin film and the beam. The substantially continuous light is required to always make one bit, and the intensity thereof is preferably selected such that a bit to be moved, which is drawn toward the bit by the temperature gradient generated from the irratiation of light is not pulled in but repelled to a predetermined distance by the repulsive force. In this case, even if the intensity of the substantially continuous light is large, the repulsive force between the bits can be produced by adjusting the distance of the position on which the light is incident from the track from which the information bit is moved.
According to the above method, it is possible to move the position of the already recorded information bit. In this case, the position to which the information bit is moved is determined by the repulsive force between the bits so that the position to which the information bit is moved is changed by a bias magnetic field. Also the position to which the information bit is moved is changed by the fluctuation of coercive force Hc, so it is difficult to stabilize the position to which the information bit is moved.
In a thermomagnetic recording system in which the thermomagnetic recording for a thin film of soft magnetic material having a low magnetic wall coercive force is carried out by heat generated from the irradiation of light thereon, the present invention is to obviate the above defects and to enable an information bit to be stably moved to a predetermined position.